This invention relates to a system for restoring a numerically controlled machine tool, especially an electric discharge machine, to a former condition. More particularly, the invention relates to a system which allows a machine tool to be restored to the condition of the machine prior to an interruption in power, such as a power failure, after power has been resupplied to the machine.
So-called wire-cut electric discharge machines are well-known in the art. In such machines a wire electrode is tensioned between an upper guide and lower guide, and an electric discharge is produced between the wire electrode and a workpiece placed on a movable table. By moving the table along the X and Y axes following an instructed machining path, the workpiece is transported with respect to the wire electrode so that the wire may cut the workpiece as instructed. When the wire electrode tensioned between the upper and lower guides is retained so as to lie perpendicular to the workpiece, the upper and lower surfaces of the workpiece can be cut into the same shape. It is possible also to adopt an arrangement in which the upper guide can be displaced along the X and Y axes, such as in a direction at right angles to the direction in which the workpiece is travelling, to incline the wire electrode with respect to the workpiece surface. This causes the upper and lower surfaces of the workpiece to be cut differently, enabling so-called taper cutting.
A discharge machining operation performed by the wire-cut discharge machine of the aforesaid type can continue for an extremely long period of time, even for as long as one week in some cases. The workpieces which are cut by electric discharge machining generally are mold materials and the like, and some materials can be extremely expensive. Since an interruption in power can occur during an extended machining period, such as by an operator inadvertently opening a switch or as the result of a power failure, it is desired that cutting be resumed, as soon as power is restored, from the position formerly occupied by the wire electrode prior to the power interruption. Without out such an arrangement the workpiece would have to be discarded and a new workpiece machined from the beginning. For a long-term electric discharge machining operation of up to one week, a power failure can of course occur at night in an unattended plant. A system which is capable of restarting the cutting operation should therefore be able to do so automatically without operator intervention after power is restored.
When power is interrupted certain phenomena occur which make it difficult to realize the desired system. For example, there is destruction of current position and commanded position information with regard to a drive motor and a movable member such as the work table, and destruction of positional control information relating to backlash direction, pitch error compensation number and the like. There is also destruction of interpolation control information, such as may be stored in the counters of a numerical control device, indicative of the process steps executed up until power interruption. Such information may be the interpolation pulse number or the block number, counting from the beginning of the numerical control command data. Another complicating factor is the inertia possessed by the table or motor when power is lost. Such inertia can carry the table over small distances on the order of several microns.
Conventional arrangements have not been able to restart electric discharge machining rapidly from the position occupied before the power interruption, and needless to say they have not been able to restore machining automatically following the return of power. In conventional practice, therefore, the following tedious and complicated procedure must be followed to resume a discharge machining operation:
(1) The discharge machining starting point is stored in advance. For example, this might be set on a digital switch or stored in a non-volatile memory.
(2) Following the restoration of power the wire electrode is removed and the table, or the upper and lower guides which guide the wire electrode, is returned to the zero point of the machine. This brings the position of the table or of the upper and lower wire electrode guides into coincidence with the current position stored in a volatile memory, cleared when power is restored, located within the numerical control device.
(3) Following the return to the machine zero point the table, or the upper and lower wire electrode guides, is positioned at the machining starting point. This is accomplished by using the aforesaid machining starting point information stored previously in the non-volatile memory or set on the digital switch in step (1) above.
(4) Upon completion of the machining starting point positioning operation, the wire electrode is again disposed on the upper and lower guides and tensioned, and the operator returns to the beginning of the numerical control command data.
(5) Next, starting from the data at the beginning of the numerical control command data, the table or wire electrode is transported, as a speed higher than the originally commanded speed, along the path previously machined in accordance with the program. Here a familiar dry-run function can be employed to transport the table or wire electrode at the higher speed.
(6) When the table or wire electrode has reached a position just short of that occupied at the time of the power interruption, the feed speed of the table is restored to the programmed ordinary feed speed for a cutting operation. Then the machining power source is turned on to introduce an electric current to the wire electrode which now starts to cut through the workpiece.
It is obvious from the foregoing that processing following the reintroduction of power is complicated and that a considerable period of time is required to restore the cutting operation. The result is a marked decrease in machining efficiency. Furthermore, since human intervension is necessary, an interruption in machining caused by a power failure which occurs at night or during a holiday will not be remedied until the next working day. Again the obvious result is a decline in efficiency.